The present disclosure relates to a nitride semiconductor light-emitting device including a semiconductor light-emitting chip held on a mounting surface of a mounting substrate, having a growth surface that is a nonpolar or semipolar plane, and emitting polarized light, a reflection member having a reflective surface off which the polarized light is reflected, and a coupler holding the reflection member.
Nitride semiconductors containing nitrogen (N) as a group V element have been expected as a material of a short wavelength light-emitting element because of their band gap size. Gallium nitride-based compound semiconductors, in particular, have been actively researched, and blue light-emitting diodes (LEDs), green LEDs, and blue semiconductor laser diodes that use a gallium nitride-based compound semiconductor have been also commercialized.
Gallium nitride-based compound semiconductors include a compound semiconductor obtained by substituting at least one of aluminum (Al) or indium (In) for part of gallium (Ga). Such a nitride semiconductor is represented by the general formula AlxGayInzN (where 0≦x<1, 0<y≦1, 0≦z<1, and x+y+z=1). The gallium nitride-based compound semiconductors are hereinafter referred to as GaN-based semiconductors.
The replacement of Ga atoms with Al atoms in a GaN-based semiconductor allows the band gap of the GaN-based semiconductor to be wider than that of GaN, and the replacement of Ga atoms with In atoms in a GaN-based semiconductor allows the band gap of the GaN-based semiconductor to be narrower than that of GaN. Thus, not only short wavelength light, such as blue or green light, but also long wavelength light, such as orange or red light, can be emitted. From such a feature, nitride semiconductor light-emitting elements have been expected to be used for, e.g., image display devices and lighting devices.
Nitride semiconductors have a wurtzite crystal structure. In FIGS. 1A, 1B, and 1C, the plane orientations of the wurtzite crystal structure are expressed in four-index notation (hexagonal indices). In four-index notation, crystal planes and the orientations of the planes are expressed using primitive vectors expressed as a1, a2, a3, and c. The primitive vector c extends in a [0001] direction, and an axis in this direction is referred to as a “c-axis.” A plane perpendicular to the c-axis is referred to as a “c-plane” or a “(0001) plane.” FIG. 1A illustrates, not only the c-plane, but also an a-plane (=(11-20) plane) and an m-plane (=(1-100) plane). FIG. 1B illustrates an r-plane (=(1-102) plane), and FIG. 1C illustrates a (11-22) plane. Herein, the symbol “−” attached to the left of one of parenthesized numbers indicating the Miller indices expediently indicates inversion of the number.
FIG. 2A illustrates a crystal structure of a GaN-based semiconductor using a ball-and-stick model. FIG. 2B is a ball-and-stick model obtained by observing atomic arrangement in the vicinity of the m-plane surface from an a-axis direction. The m-plane is perpendicular to the plane of the paper of FIG. 2B. FIG. 2C is a ball-and-stick model obtained by observing atomic arrangement of a +c-plane surface from an m-axis direction. The c-plane is perpendicular to the plane of the paper of FIG. 2C. As seen from FIGS. 2A and 2B, N atoms and Ga atoms are located on a plane parallel to the m-plane. On the other hand, as seen from FIGS. 2A and 2C, a layer in which only Ga atoms are located, and a layer in which only N atoms are located are formed on the c-plane.
Conventionally, when a semiconductor element is to be fabricated using a GaN-based semiconductor, a c-plane substrate, i.e., a substrate having a (0001) plane as its principal surface, has been used as a substrate on which a nitride semiconductor crystal is grown. In this case, spontaneous electrical polarization is induced in the nitride semiconductor along the c-axis due to the arrangements of Ga and N atoms. Thus, the “c-plane” is referred to as a “polar plane.” As a result of the electrical polarization, a piezoelectric field is generated in a quantum well layer forming a portion of a light-emitting layer of a nitride semiconductor light-emitting element and made of InGaN along the c-axis. Due to the generated piezoelectric field, the distributed electrons and holes in the light-emitting layer are displaced, and the internal quantum efficiency of the light-emitting layer is decreased due to a quantum-confined Stark effect of carriers. In order to reduce the decrease in the internal quantum efficiency of the light-emitting layer, the light-emitting layer formed on the (0001) plane is designed to have a thickness equal to or less than 3 nm.
Furthermore, in recent years, consideration has been made to fabricate a light-emitting element using a substrate having an m- or a-plane called a nonpolar plane, or a −r- or (11-22) plane called a semipolar plane as its principal surface. As illustrated in FIG. 1A, m-planes of the wurtzite crystal structure are parallel to the c-axis, and are six equivalent planes orthogonal to the c-plane. For example, in FIG. 1A, a (1-100) plane perpendicular to a [1-100] direction corresponds to one of the m-planes. The other m-planes equivalent to the (1-100) plane include a (−1010) plane, a (10-10) plane, a (−1100) plane, a (01-10) plane, and a (0-110) plane.
As illustrated in FIGS. 2A and 2B, Ga and N atoms on the m-planes are present on the same atomic plane, and thus, electrical polarization is not induced in directions perpendicular to the m-planes. Therefore, when a light-emitting element is fabricated using a semiconductor stacked structure having an m-plane as its growth surface, a piezoelectric field is not generated in a light-emitting layer, and the problem where the internal quantum efficiency is decreased due to the quantum-confined Stark effect of carriers can be solved. This applies also to the a-plane that is a nonpolar plane except the m-planes, and furthermore, even when, instead of the m-plane, the −r- or (11-22) plane called the semipolar plane is used as the growth surface, similar advantages can be provided.
A nitride semiconductor light-emitting element including an active layer having an m- or a-plane, or a −r- or (11-22) plane as a growth surface has polarization characteristics resulting from the structure of the valence band of the active layer.
For example, Japanese Unexamined Patent Publication No. 2008-109098 describes a light-emitting diode device including light-emitting diode chips 10 each including a light-emitting layer 12 having a principal plane 12a, and a package 20 having a chip-arrangement surface 21a on which the light-emitting diode chips 10 are arranged, and configured such that light emitted from the principal plane 12a of the light-emitting layer 12 has a plurality of different intensities depending on the in-plane azimuth angles of the light in the principal plane 12a of the light-emitting layer 12, and at least either the light-emitting diode chips 10 or the package 20 reduces variations in the intensity of light exiting from the package 20 due to the differences among the in-plane azimuth angles of the light in the chip-arrangement surface 21a, in order to reduce the variations in the intensity of light exiting from the package due to the differences among the in-plane azimuth angles of the light in the chip-arrangement surface.
Japanese Unexamined Patent Publication No. 2009-38293 describes a light-emitting device configured such that, in order to prevent diffusion of polarized light, at least part of an inner surface of a mounting base on which a light-emitting element is mounted forms a specular surface.
Japanese Unexamined Patent Publication No. 2009-88353 describes a light-emitting device including a light-emitting element, and a package in order to provide a light-emitting device emitting polarized light with a high polarization ratio. The light-emitting element has a first end surface from which first polarized light is emitted, and a second end surface from which second polarized light is emitted. The package has a first inner wall surface that faces the first end surface and extends in parallel with the first end surface, and a second inner wall surface off which the second polarized light is reflected toward the first inner wall surface.